The present invention relates to an improved magneto for an engine ignition system. More particularly, the invention relates to a magneto which generates first and second induce voltage waveforms wherein the phase of the second induced voltage waveform varies relative to the first induced voltage waveform in proportion to the speed of the engine.
The magneto of this invention is particularly useful with small engines of the type commonly used in chain saws, snow blowers, portable power generators, lawn mowers and the like. While the preferred embodiment of the invention is described for use with a two cylinder engine, this is not a limitation of the invention in that the invention may be extended to multi-cylinder engines by incorporation of a distributor of a know type or use a plurality of magnetos with each of the cylinders.
Magnetos for use with ignition systems such as capacitive discharge or inductive systems have used various coil and core arrangements positioned adjacent a moving rotor which includes a magnet attached to and movable with the engine flywheel or fan. As the rotor passes the core or stator various voltages are induced in the coils. One such device incorporating an E-type stator supporting a plurality of coils is shown in my commonly assigned U.S. Pat. No. 4,509,493 which issued Apr. 8, 1985.
The magneto of the illustrated embodiment of my invention generates a plurality of AC voltage waveforms. More particularly, the magneto generates first and second voltage waveform which include first and a second trigger pulses. The uniqueness of the present invention is that the second trigger pulse of the second waveform automatically advances in time relative to the generation of the second trigger pulse of the first waveform as a function of engine speed. By properly spacing the coils embedded in the stator the second trigger pulse of the second waveform can be controlled such that at a predetermined nominal engine speed its phase will have advanced a sufficient amount such that this second trigger pulse occurs simultaneously with or slightly before the second trigger pulse of the first waveform. This variable phase or walking phenomena can be sensed by associated control circuitry to automatically change the ignition spark timing from retard to advance or used for other purposes. Once coil spacing has been established, based upon the charcteristics of the particular engine in use no further calibration is required.
It is an object to provide a simply constructed magneto capable of generating a walking or phase variable waveform. A further object of the invention is to provide a magneto which solves the deficiencies in the prior art. Most prior magneto control circuits make use of the change in amplitude of signals that occur as flywheel speed changes. The deficiency of these systems is in that the control circuitry responds to other conditions that affect signal amplitude. These circuits typically require calibration on the assembly line to accommodate the variations in electronic components, which results in high labor costs, and still vary in performance when installed on an engine because of changes in temperature, changes in module to magnet air gap, changes in electronic components as they age, etc. My invention is insensitive to signal amplitudes and responds only to the distance between two windings (which is fixed upon manufacture) and flywheel speed.
Accordingly the present inventions comprises:
A magneto for an engine ignition system comprising means for generating a first induced voltage; and means for generating a second induced voltage having a phase which varies relative to said first induced voltage in proportion to the speed of the engine. More particularly, the magneto comprises means for generating a first induced voltage characterized by a first portion or first trigger pulse of a first polarity, followed by a second portion of opposite polarity and a third portion or second trigger pulse of the first polarity; and means for generating a second induced voltage characterized by a first portion or first trigger pulse of the first polarity followed by a second portion of the opposite polarity, and a third portion or second trigger pulse having a phase which varies relative to the second trigger pulse of the first induced voltage in proportion to the speed of the engine. The magneto also includes means for generating a third induced voltage of polarity opposite to the polarity of the first induced voltage. More specifically the magneto comprises an E-type pickup or stator comprising a core including three parallel extending poles which can be asymmetrically spaced terminating at end faces; and a rotor comprising permanent magnet means for producing a varying flux field in the stator, adapted to be moved across the end faces to produce the varying flux field.
When using asymmetrically spaced poles the poles define a first spacing between a second or center pole and a first pole and a larger second spacing between the center pole and a third pole. The stator is positioned relative to the rotor such that the rotor moves across the stator from the first pole toward the third pole. The magneto further includes a first or charge coil and a second or trigger coil both wound about the center pole. A third or suppression coil is wound about a ferromagnetic core and positioned in the second spacing remote from the core preferably though not necessarily proximate the end face of the center pole and electrically isolated, from the trigger coil.
Many other objects, advantages and purposes of the invention will be clear from the detailed description of the drawings.